This invention relates generally to non-complementary or single ended noise reduction systems and more particularly concerns non-complementary noise reduction systems incorporating dynamically controlled filtering and low level downward expansion.
Present audio technology is such that even modest systems are capable of accurate reproduction over the entire audio bandwidth. Consequently, background noise presents a problem in almost every audio application, including tape and disc recording and playback, radio and television broadcasting and electronic musical instrument and signal processing.
The relationship of dynamic range to noise reduction in both live and recorded material is of growing significance. Noise awareness is dramatically increased when little or no signal is present to mask the noise within a given portion of the audio spectrum. Dynamically controlled low pass and high pass filters have long been used to reduce noise in high and low segments of the audio spectrum. One such system presently in use applies only the principles of dynamically controlled low pass filtering and provides no reduction of noise in the low frequency portion of the audio spectrum. Others use both low and high pass filtering and thus narrow the bandwidth to mid-audio range frequencies, but this still permits noise to be carried through the system which would be audible in the absence of mid-frequency information.
Another known method of noise reduction is the use of low level expansion to reduce the overall level of the audio when the signal level drops below a predetermined point. Such systems provide no noise reduction at higher signal levels. Furthermore they are generally more responsive to low level transient surges, thus accentuating the pumping or breathing effect in these systems. Consequently, many of these systems employ elaborate circuitry to detect and compensate for the low level surges.
Systems do exist which utilize both dynamically controlled variable bandwidth low pass filtering and low level downward expansion. These existing systems, however, suffer from a wide range of problems or side effects. The range of response in the filter section is generally limited. That is, at low signal levels the amplitude of the signal is not sufficient to open the dynamic filter to pass all of the high frequency audio information, resulting in a severe loss of high frequency response. Furthermore, a 6dB per octave filter is generally used to limit the amount of attenuation in the high frequency spectrum, but when signal levels exceed the optimal operating level the dynamic filter is held open at all times. Thus, no noise reduction is obtained at high signal levels and at times a breathing effect is produced. Also in these systems, the downward expander is designed to produce no downward expansion when the signal is above a preselected threshold level. When the signal level drops below this point, expansion occurs. Consequently, percussive or other high transient material will modulate both the noise floor and the audio signal, causing pumping and breathing effects.